During an HIV-infection there is a loss of T cells with an increase in production and repopulation of the lymphoid system with new T cells. With age there is a decline in the ability to replace the T cells lost in the disease process. Younger HIV- infected patients can experience extended periods where there CD4+ cell level remains in the normal range and they are free of opportunistic infections. The mechanisms accounting for the T cell loss are still unclear. They are thought to be linked to an autoimmune process and/or an apoptotic event during cellular activation. Both mechanisms play a role. The immune response to HIV involves both anti-HIV antibody as well as anti-HLA antibody. This suggests the possibility that individuals with some HLA haplotypes may be more resistant to the effects of an HIV infection especially since genetics plays a role in resistance to infectious illness as well as to the immune response. Furthermore, genetic manipulation of cell lines allows the cells to resist the detrimental effects of the HIV as well as to resist infection.